1. Field of the Invention
This invention relates to a lanthanum beryllate laser that exhibits reduced thermal lensing, because its laser medium is cut with one of a certain family of crystal orientations.
2. Description of the Prior Art
A number of solids, both crystals and glasses, have been found to be suitable for laser action since the first (solid-state) laser was demonstrated by Maiman in 1960. Generally, the laser-active materials involve a rare earth, actinide, or transition metal dopant in a crystalline or glass host. An extensive treatment of then-known solid-state lasers was published in 1976-Solid-State Laser Engineering, W. Koechner, Springer-Verlag, New York. More recently, a compilation of laser crystals was presented in Laser Crystals, A. A. Kaminskii, Springer-Verlag, New York (1981). (See also P. Moulton, Laser Focus, May 1983, pp. 83 ff)
Among solid-state laser host materials is lanthanum beryllate (BEL), which was disclosed in U.S. Pat. Nos. 3,866,142, issued Feb. 11, 1975 and 3,983,051, issued Sept. 28, 1976. The specific laser materials disclosed in those patents are described by the formula Be.sub.2 La.sub.2-2x Z.sub.2x O.sub.5, where Z is a dopant selected from the group consisting of praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and mixtures thereof and where x is a positive value not greater than about 0.2. Nd:BEL is a preferred composition.
BEL crystallizes in the monoclinic system (space group C2/c) with a structure based on a three dimensional framework of corner sharing BeO.sub.4 tetrahedra. The interstices of the framework form La.sub.3+ sites (point group C1). The structure bears some resemblance to the coesite form quartz and to the feldspars, which are SiO.sub.4 framework structures, but differs in that each tetrahedron in BEL has one unshared corner. The lattice constants are a.sub.O =0.753 nm, b.sub.O =0.734 nm, and c.sub.O =0.744 nm; b is the two-fold rotation axis and the angle between the a and c axes is 91.degree.33'. As a consequence of the monoclinic structure, BEL is optically biaxial.
The relationship between the mutually orthogonal principal vibration directions, X, Y, and Z, and the crystallographic directions a, b, and c, is shown in FIG. 1. The optical vibration direction Y coincides with the crystallographic 2-fold rotation axis b. The optical X and Z directions lie in the crystallographic a-c plane. In this plane c and Z are related by .theta.=31.7.degree. at .lambda.=1 .mu.m. The refractive indices at .lambda.=1.00 .mu.m are 1.964, .beta.=1.997 and .gamma.=2.035 X-, Y-, and Z-polarized light, respectively.
Optical pumping of a laser rod leads to a substantially radial thermal gradient in the rod, which in turn gives rise to "thermal lensing". (See Koechner op. cit., pp. 352 ff) Thermal lensing is a distortion of the laser beam that results from three separate contributions: thermal expansion causes elastic distortion of the laser rod, resulting in the fomration of a lens. The stress optic lens results from the change in refractive index with thermally-induced stresses. The third contribution, which is by far the largest of the three in BEL, is the temperature dependence of the refractive index, dn/dT. This is the contribution that is primarily addressed by this invention.
An analysis of the thermal and mechanical properties of Nd:BEL, as they relate to its use as a laser medium, was described in a report "Repetitively Q-switched Nd:BeL Lasers," prepared for Goddard Space Flight Center by the Aerospace Corporation. The report describes the techniques by which the authors measured the coefficient of linear thermal expansion and the temperature dependence of the refractive index, dn/dT. In addition to reporting the results, the report suggests that thermal lensing in Nd:BEL could be eliminated by choosing a geometric aspect ratio (rod diameter/rod length) of 0.625, for which the lens induced by positive thermal expansion cancels the lens induced by negative dn/dT. That aspect ratio is six times that for a "typical laser rod," and such a large aspect ratio is not desirable for a laser rod. It is hard to pump efficiently and has a low gain, because of the relatively short path of the beam in traversing the crystal. Furthermore, for that rod to be athermal it must be pumped along its entire length, which makes mounting difficult.
Another laser material of interest is holmium doped lithium yttrium fluoride, which was disclosed in U.S. Pat. No. 4,110,702, issued Aug. 29, 1978.
In an effort to minimize thermal distortion of laser beams, rectangular cross-section slabs have been used in place of cylindrical laser rods. A rectangular slab provides a larger cooling surface and can yield a one-dimensional temperature gradient across its thickness. If, in addition, the laser beam is made to pass through the slab in zig-zag fashion (by total internal reflection), optical distortion is minimized, because each ray in the beam passes through the same refractive index variation, and there is, thus, minimal wavefront distortion. The zig-zag slab can generally be used with any solid-state laser medium; however, the zig-zag slab laser requires that the opposing faces be optically polished, flat and parallel to optical wavelength tolerances, and not in contact with anything, such as a support, that would interfere with the total internal reflection.